The present invention is generally in the field of suspensions of polymer particles, and more particularly to methods for producing suspensions of amorphous polymer particles, especially of polyhydroxyalkanoate polymers.
Polyhydroxyalkanoates (PRAs) are useful biodegradable thermoplastics obtained from microbes or plants or synthesized chemically. Although typically biological in origin, PHAs may be processed like conventional plastics to produce coatings, films, and molded objects. A particularly useful form of PHAs is a suspension including polymer particles suspended in water, wherein a majority of the particles are in an amorphous, i.e. noncrystalline, state. These suspensions have application in ambient temperature film formation, since the glass transition temperature (T.sub.g) of PHAs is generally less than 5.degree. C. Amorphous PHA suspensions (which also are referred to as amorphous latexes or emulsions) have been found to be particularly useful in, for example, architectural paints (GB 2,291,648), cathode-ray tube metallization lacquers (PCT/GB95/02761), and coatings for paper (U.S. Pat. No. 5,451,456), cellulose or nonwoven objects (PCT/GB96/00284), and foods (PCT/NL95/00222).
Amorphous PHA particles can remain noncrystalline almost indefinitely, even when cooled 100.degree. C. or more below the crystalline melting temperature, provided that the particles are sufficiently small and behave independently from each other, as they do in aqueous suspension (Horowitz, et al., J. Am. Chem. Soc. 116:2695-702 (1994)). They remain amorphous even if the temperature is maintained well above T.sub.g, conditions at which the bulk polymer would crystallize rapidly. This unexpected stability in the amorphous state has been explained based on nucleation kinetics (de Koning & Lemstra, Polymer 33:3292-94 (1992)). The theoretical model predicts that the observed rate of crystallization for an ensemble of polymer particles varies inversely with the particle volume, i.e. with the third power of particle diameter. The half-time for the crystallization process accordingly can be expressed as (log.sub.e 2)/Iv, where I is the nucleation rate constant and v is the volume of an individual particle. Using known nucleation rate constant data (Organ & Barham, J. Mater. Sci., 26:1368-74 (1991)), the half-times for crystallization of suspensions of various amorphous PHA particle sizes for poly-3-hydroxybutyrate (PHB) and poly-3-hydroxybutyrate-co-7%-hydroxyvalerate (PHBV) (BIOPOL.TM.) at 30.degree. C. have been calculated and are shown in Table 1 below.
TABLE 1 Predicted Half-times for Crystallization of Suspensions of Amorphous PHA Particles Particle Size PHB PHBV 100 .mu.m 9 min 44 min 20 .mu.m 18 hours 92 hours 5.0 .mu.m 49 days 245 days 1.0 .mu.m 17 years 84 years 0.2 .mu.m 2099 years 10494 years
Although amorphous particles are quite stable in a water suspension, when the suspension is applied to a surface and allowed to dry, the particles coalesce to form a coherent or semi-coherent crystalline film having desirable functional properties. Despite the usefulness of such PHA suspensions for forming biodegradable, hydrolyzable, or thermolyzable films, it has generally proven difficult to prepare these materials on a commercial scale.
In the laboratory, amorphous PHA suspensions have been prepared from crystalline polymer by solvent emulsification routes. For example, the PHAs poly-3-hydroxybutyrate (PB), poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PBBV or BIOPOL.TM.), and poly-3-hydroxyoctanoate (PHO) have been prepared as amorphous particles by dissolving the polymer in chloroform, emulsifying the polymer solution with a solution of aqueous surfactant, and heating the emulsion to remove the organic solvent (U.S. Pat. No. 5,599,891; Horowitz & Sanders, Polymer 35:5079-84 (1994)). Suspensions of amorphous PHO particles also have been prepared by dispersing an acetone solution of the polymer in water, and then removing the acetone by distillation .(Marchessault, et al., Can. J. Microbiol. 41:138-42 (1995)). A similar approach is described in PCT/GB96/00305 by Liddell, et al., in which a suspension of amorphous PHBV is made by mixing a solution of polymer in propanediol at high temperature with water and surfactant, and then removing the organic solvent by diafiltration. The disadvantage, however, of all of the foregoing methods is that the particle suspension is highly diluted, necessitating extensive concentration at the end of the process and resulting in a low throughput of material. In addition, an organic solvent is used and removed in the process, which necessitates the use of costly solvent recovery and purification equipment when conducted on a commercial scale.
PCT/GB95/01925 by Liddell, et al. also discloses a method for producing a "PHBV latex" by heating a slurry of crystalline PHBV powder in water and detergent under pressure for ten minutes at a temperature above the melting temperature of the polymer. It is not disclosed whether the particles remained in the amorphous state upon cooling, or what the final particle size of the slurry is. This method, however, has the disadvantage that the particle size of the final product is dictated by the size of the powder starting material, which is at least 20-50 .mu.m. Particles of this size crystallize rapidly upon cooling (see Table 1 above), giving even an initially amorphous suspension a useful shelf-life of less than four days. The duration of the thermal treatment in this case also causes significant further agglomeration of the particles. Suspensions containing large particles are generally unsuitable for film forming applications.
It is generally preferable to purify PHAs using aqueous (i.e. organic solvent-free) methods, particularly in commercial scale processes, in which the polymer remains in a microparticulate state and the non-PHA biomass is solubilized through mechanical, chemical, and/or enzymatic treatments. The PHA particles then are separated from the solubilized material using centrifugation, filtration, floatation, or other convenient methods. For example, EP 0 145 233 discloses aqueous methods for purifying a microbiological 3-hydroxybutyrate polymer wherein the cells are heat-treated at above 80.degree. C. and then digested with enzymes, surfactants, and/or hydrogen peroxide.
Although the PHA granules contained in microbial cells are completely amorphous (Barnard & Sanders, J. Biol. Chem. 264:3286-91 (1989)), the polymer suspensions obtained through aqueous harvesting methods usually have a substantial percentage of crystalline particles, which accumulate throughout the course of processing (D. M. Horowitz, Ph.D. Thesis, University of Cambridge, UK, 1994). The undesirable crystallization of the polymer particles is most pronounced with those PHAs having relatively fast crystallization kinetics, such as PHB and PHBV copolymers having 20 mol% or less 3-hydroxyvalerate content. The exact mechanism of crystallization is not fully understood; however, it is believed that certain conditions of temperature, shear, and/or contact among polymer particles or between particles and nucleating surfaces all promote crystallization at a rate well beyond that predicted by simple homogeneous nucleation kinetics. It has been found experimentally that the presence of even a modest fraction of crystalline polymer particles in the suspension results in an inferior, porous, and irregular film under ambient temperature conditions, since particle coalescence does not occur.
The undesirable crystallization that occurs during aqueous harvesting may be minimized by the use of various stabilizers, such as alkyl ethoxylates, acrylic graft copolymers, and some ionic surfactants, as disclosed, for example, in PCT/GB96/00284 and PCT/US96/1986. However, this approach can be costly and has several disadvantages. The success of the stabilizers in preventing crystallization tends to be polymer composition-dependent, with the faster crystallizing PHAs giving the poorest results. Suspensions of PHB and PHBV copolymers, for example, invariably contain some crystalline particles when processed using the stabilizers. Moreover, the most effective agents are steric stabilizers (for example, the acrylic graft copolymer Hypermer CG6, available from ICI Surfactants) which remain tightly associated with the polymer particles and have a detrimental effect on subsequent film formation.
It is therefore an object of the present invention to provide methods and apparati for producing a suspension of predominately amorphous polymer particles.
It is another object of the present invention to provide aqueous suspensions of amorphous PHA particles having a size suitable for forming PHA coatings, films, and molded objects.